The invention relates generally to cooling systems and more specifically to an electronically controlled magnetorheological fluid based cooling fan drive assembly.
Cooling systems are used on vehicles today to provide cooling to an engine during operation. Fan drives are typically driven by the engine crankshaft at a fixed ratio to cool engine coolant as it flows through a radiator. Thus, as the engine speed is reduced, as is the trend in vehicles today to reduce emissions, the fan drive speed is correspondingly reduced. Similarly, as the engine speed increases, the fan drive speed correspondingly increases. This increased fan drive speed causes the engine block temperature to cool to less than optimal levels, resulting in less than optimal conditions that can affect emissions and fuel economy.
One method used to address these issues is to add a viscous fluid coupling to drive the radiator cooling fans. In a typical viscous fluid coupling, an input shaft drives an input coupling member (clutch) which is received within an output coupling member, and torque is transmitted from the input to the output, in the presence viscous fluid, by means of viscous shear drag. The coupling normally includes some sort of valving which controls the amount of viscous fluid within a viscous shear chamber, thereby controlling the ratio of the output torque and speed to the input torque and speed. Typically, this valving comprises a valve member that is moveable to cover or uncover a fill port disposed between a reservoir and viscous shear chamber (operating chamber).
One problem with currently available viscous couplings for fan drives is the complexity of the designs. Viscous fluid must be moved from a fluid reservoir chamber to a working chamber in order to couple or uncouple the input coupling member from the output coupling member. This requires a combination of moveable valve members, valve wiper arms, and relief chambers to move the viscous fluid both into and out of the working chamber. This adds complexity and cost to the viscous coupling.
More importantly, currently available viscous couplings are either incapable of being controlled to provide instantaneously cooling to an engine block or require a period of time to increase or decrease the amount of cooling available to the engine block. This time lag may have an effect on fuel economy and emissions at various engine speeds and engine temperatures.
It is thus highly desirable to limit the complexity of the viscous coupling and provide more precise control of engine cooling capabilities when using a viscous coupling.
The above and other objects of the invention are met by the present invention that is an improvement over known viscous couplings.
The present invention discloses a magnetorheological fluid based fan drive clutch that uses a tethered stationary coil and low cost concentric drum configuration. Magnetorheological fluid, normally thin, thickens between a pair of cylindrical drums when a magnetic field is applied. This thickening allows the magnetorheological fluid to shear between the drums and transmits torque from an external shaft to an internal shaft coupled to a fan. A stationary coil mounted on a steel support housing is electrically excited to create the desired magnetic field. The amount of electrical excitation is controlled as a function of engine speed and engine block temperature to maximum fuel economy and minimize emissions at various engine temperatures and speeds.
Other features, benefits and advantages of the present invention will become apparent from the following description of the invention, when viewed in accordance with the attached drawings and appended claims.